downgraded stackage@9.21(ghc-8.0.2) due to DanielG/ghc-mod#900 (comment)

stack.yaml

@@ -17,7 +17,7 @@
 #
 # resolver: ./custom-snapshot.yaml
 # resolver: https://example.com/snapshots/2018-01-01.yaml
-resolver: lts-12.14
+resolver: lts-9.21
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.
@@ -37,7 +37,7 @@ packages:
 # Dependency packages to be pulled from upstream that are not in the resolver
 # using the same syntax as the packages field.
 # (e.g., acme-missiles-0.3)
-# extra-deps: []
+# extra-deps:
 
 # Override default flag values for local packages and extra-deps
 # flags: {}